Impedance control devices, also called tuners, are devices of which the impedance, presented to the outside world, can be changed. This is done by either manually changing a property of the device (e.g. a manual tuner, see e.g. http://www.maurymw.com/MW_RF/Manual_Tuners.php) or by changing a property via an electronic means (e.g. an automated tuner, see http://www.focus-microwaves.com). This device typically has one port or two ports but could in principal have more ports. Via the port these devices are connected to the outside world and provide controllable impedance to the outside world. In most cases, a port is a physical connector through which the impedance control device can be connected to another device. However, the port does not need to be limited to a connector. The port defines a boundary between the impedance control device and the outside world. Amongst others, a port could be a pad of an integrated circuit (IC). The impedance range that the impedance control device can provide depends on the physical properties of the device.
Impedance control devices are well established in source- and load-pull measurement set-ups or measurement systems. These set-ups are used to determine the impedances to be presented at the input and/or output of a device under test in order to optimize one or more of its performance characteristics, e.g. the delivered output power, power added efficiency and other. In this case the device under test is typically a transistor or an amplifier under test. These set-ups are also used to characterize the behaviour of devices, e.g. transistors, diodes, amplifiers, mixers etc. under realistic test conditions or to verify and/or improve their model, used in computer aided engineering tools (CAE).
The impedance control devices, which are presently used in commercially available source- and load-pull systems, are based on different techniques. As measurement means, these source- and load-pull systems use different types of measurement receivers: one or more power meter, spectrum analyzer, network analyzer, oscilloscope, . . . .
First, there are the passive impedance control devices. They are based on one or more moveable resonator or slug. These tuners usually are bulky due to the mechanical aspects, while shrinking in size with increasing frequency due to reduced wavelength. As such they take a lot of space in a measurement set-up. To move the resonator(s) or slug(s) automatically, tuners contain step motors. As these parts are moving to synthesize a new impedance, the tuners can cause vibrations in the measurement set-up. This is typically a problem for on-wafer measurements. The use of pin-diodes, positioned at different positions of transmission line stubs, has been an alternative to synthesize impedances in a passive way. This approach results in smaller form factors, eliminating the step motors, but is presently limited to power levels up to approximately 35-40 dBm. The principle is based on creating reflections on a transmission line at different positions by turning on or off the pin diodes at these positions. As such the size is dependent on the frequency range. Pin diodes can be switched on and off very fast. Consequently impedances can be tuned fast. This type of tuners has never been integrated into other functional modules, like couplers, in spite of the smaller form factor. Secondly, there are the active tuners with different types of closed loop control. They sense the output power of the DUT, amplify or attenuate it and shift it in phase and re-inject this signal towards the device under test as reflected wave. Meanwhile proper selection in topology and narrowband filtering in the loop minimizes the risk of oscillation. These set-ups usually are also bulky because the couplers, amplifiers, attenuators, filters and phase shifters are connectorized devices. Thirdly, there are the active tuners with open loop control. They actively inject power towards the DUT output in a phase coherent way with the source which provides the input signal to the device under test. This can be realized in different ways, e.g. by splitting the input source, followed by amplifying or attenuating and phase shifting it (as in the presentation “Active and passive load-pull systems: from the basic to the future of variable impedance device characterization”, A. Ferrero et al, PAF, pp. 13-14, IMS 2005 Workshop WSG), or by using a second source which is controllable in amplitude and phase and phase locked to the source at the input (see “High power active harmonic load-pull system for characterization of high power 100-watt transistors”, Z. Aboush et al., EUMC 2005, Proc. Vol. 1). For both approaches, the signal injected back to the device under test is amplified or attenuated and controlled in phase compared to the signal that comes out of the device under test. In this way synthetically different impedances can be synthesized. A similar approach can be used to synthesize impedances at the input of the device under test, typically at harmonics of the input source. Also this set-up is bulky requiring splitters, possibly a second source, amplifiers, phase control, possibly filters etc. . . . .
Due to the mechanical dimensions of passive tuners and the length of cable and the parts used for the active tuning, there is usually a meaningful signal delay between the plane of the device under test, where one wants to synthesize an impedance, and the place where the actual tuning happens. For broadband modulation signals the impedance at the device under test will not be constant across the modulation bandwidth and will deviate from the impedance synthesized at a given frequency within that band.
The speed of passive tuners, except for the tuners based on pin diodes, is related to the step motors speed and the inertia of the mechanical structure. The active tuners need to track all the time the input signal to maintain the impedance at the output constant.
Due to their precise construction, passive tuners, and as such tuner set-ups, are usually quite expensive. Also active tuners are usually quite expensive due to the required additional hardware.
Commercially available set-ups, provided with an impedance control device, minimally contain a source to stimulate the device under test, the DUT itself, followed by a tuner and a means (e.g. a power meter) to measure the power transmitted by the DUT under different impedance conditions, as illustrated in FIG. 1.
Further extended set-ups also use power measurement capability at the input to measure input power, possibly in combination with a source tuner and the capability to measure reflected power at the input, a spectrum analyzer at the output to perform frequency selective power measurements and to monitor stability (FIG. 2).
If one wants to measure more information at the DUT, it is possible to use a vector network analyser, an oscilloscope or a receiver with similar capabilities in combination with signal separation hardware that can detect samples of the incident and reflected waves (or a combination thereof). The selected receivers allow measuring in a frequency selective way a derivate of the incident and reflected waves or the voltage and the current at the ports of the device under test. With the most advanced systems today it is possible to measure both amplitude and phase of the spectral components present in said derivate of the incident and reflected waves (FIG. 3). Thanks to absolute calibration techniques it is possible to relate the derived quantities to the incident and reflected waves or voltage and currents in the calibration plane.
Signal separation should be construed in its broadest sense. In FIG. 1 the signal separation is just a signal path connecting the tuner to the power sensor. In FIG. 2 the signal separation is the hardware that probes one quantity, e.g. an incident or reflected wave or a voltage or a current. The signal separation hardware can also be arranged to probe two signal quantities, e.g. an incident and reflected wave (one bidirectional coupler (as in FIG. 3) or two unidirectional couplers) or voltage and current (voltage and current probes) or a combination thereof. For the signal separation hardware possibly distributed couplers, loop couplers or IV probes are being used. They can be single or dual depending on the number of quantities being measured.
In set-ups to measure the incident and reflected waves or a combination thereof, typically in a frequency-selective way, the signal separation hardware can be put (see FIG. 3) outside the combination of device under test and tuner (after the DUT and tuner) or between the DUT and the tuner (FIG. 4). With the first configuration (FIG. 3) one needs to use the S-parameters of the tuner to properly de-embed the measurements up to the device under test as the impedance is being changed. With the second configuration (FIG. 4) the incident and reflected waves or a combination thereof are always measured at the DUT, independently of the tuner position. The accuracy is related to the used calibration technique. Also in case of the synthesis of reflection factors close to the edge of the Smith chart, this approach (FIG. 4) is the only viable approach. In this case, one needs to select signal separation hardware that minimizes the losses between the DUT and the tuner, as the losses do reduce the coverage area of the Smith chart. Due to the advantages of the latter set-up and the need to minimize the losses, signal separation hardware has been integrated into the tuners (see for example U.S. Pat. No. 7,548,069 and U.S. Pat. No. 7,282,926). This is possible thanks to the large size of the impedance control devices.
Presently the passive tuners or impedance control devices are large such that they take meaningful space on a measurement bench and create practical problems to combine with on wafer measurements (e.g. space and vibration during the movement of the mechanical tuner parts). Further, they are also rather heavy and difficult to bring very close to the DUT. Especially on wafer this creates mechanical challenges. It is further to be noted that due to their principle of operation, the dimensions of the passive tuner increase when the frequency of interest lowers. Because of their size, passive tuners are very difficult to integrate with test and measurement instrumentation. As mentioned, also the source- and load-pull systems based on active tuners suffer from their large size and with both approaches it is very difficult to realize impedances, which are enough constant for a broadband modulation.
Hence, there is a need for overcoming these drawbacks.